Add semantic diff explainer

[orban]

Summary

• add scripts/explain_semantic_diff.sh as a dedicated CLI that explains the semantic meaning of a diff per affected Intent node
• keep the first version shell-native and deterministic offline, with optional Claude-backed enrichment when ANTHROPIC_API_KEY is present
• add coverage for help, invalid refs, no-change handling, dry-run output, affected-node detection, and representative semantic formatting

Scope

This PR intentionally stays narrow. It does not refactor the agent system or eval harness. Instead it adds a standalone workflow that sits between detect_changes.sh and suggest_updates.sh:

• detect_changes.sh answers which nodes changed
• explain_semantic_diff.sh answers what those changes likely mean semantically
• suggest_updates.sh answers what to write back into nodes

Assumptions / Limitations

• offline mode uses heuristics, so summaries are directionally useful but not authoritative
• the Claude path is optional and filters sensitive file diffs before prompting, but it still inherits the usual LLM interpretation risk
• untracked files are not included, matching the existing git-diff based scripts

Tests

• ./tests/test_explain_semantic_diff.sh
• ./tests/test_suggest_updates.sh

Sample Output

# Semantic Diff Explainer

Range: main..HEAD
Changed files: 2

## src/AGENTS.md

Summary: The diff changes executable code in this node and deserves semantic review.
Changed files: 2
Confidence: high

Behavioral impact: Behavior likely changed in code paths covered by this node, not just formatting or comments.
Contract impact: Interfaces, invariants, or caller expectations likely changed and should be reviewed against the node guidance.
Internal-only signal: No clear internal-only signal.

Review focus:
- Re-read the node entry point `src/api.ts` because the main workflow changed.
- Inspect `src/api.ts` for user-visible logic changes and edge-case handling.
- Check whether test updates in `tests/api.test.ts` reflect intended behavior rather than masking a regression.

[orban]

Superseded by #27 (via #26). Closing to consolidate review on the latest iteration.